Electroluminescence is the emission of light from an electroactive material upon the application of a sufficient voltage to cause charge carrier (hole/electron) injection at the electrode (anode/cathode). Charge carrier injection is followed by charge migration through the electroactive material under the influence of an applied electric field. When the holes and electrons meet, they recombine, emitting light. The color of the light emitted depends on the bandgap (or HOMO-LUMO gap) of the material and can span the entire visible region of the spectrum. Many luminescent displays, such as those in PDAs and cell phones that contain OLED displays, operate in this manner. Another luminescent display is the light-emitting electrochemical cell (LEC). This device differs in that it contains ionic species in the electroactive layer that help lower the barrier for charge injection, allowing for the use of non-reactive electrode materials.
Another type of display for consumer electronics (e-books, electronic paper, reflective LCDs) is a reflective display, which can be based on electrochromics. Electrochromism is the change in color of an electroactive material induced by the reduction or oxidation of an electrochrome at an electrode surface. This color change can be between multiple color states or from a colored state to a transmissive state. Typical electrochromic displays contain an electroactive, electrochromic material at a working electrode and at a counter electrode where the electrodes are in contact with an interdisposed electrolyte. These displays can be of a transmissive window-type, where both electrodes are transmissive, or of a reflective mirror-type, where one electrode is reflective.
For consumer electronic devices (e.g., cell phones, PDAs, mp3 players, and digital cameras), the displays typically are one of three types: reflective displays (i.e., reflective LCDs, IMOD, e-paper); transmission (i.e., transmissive LCDs); or light emission (i.e., OLEDs). One drawback exhibited by such displays is that the device is often used in a variety of environments with varying degrees of ambient light. For the transmissive or emissive displays, outdoor ambient lighting frequently overwhelms the display back-light resulting in a washed-out image. In contrast, low ambient lighting does not allow sufficient contrast with a reflective display without the use of an added light source. Hence, a display that can effectively function in a variety of lighting conditions by switching between a full color reflective electrochromic operation and a light-emitting operation would be advantageous.